This invention relates to a method of braking a vehicle in which a braking force is applied to ground wheels.
More particularly but not exclusively the invention relates to a method of braking large wheels on vehicles, for examples, a tractor or loading and/or excavating machine. By “wheels” we include the wheel hub and tyre mounted on the hub.
Such large wheel vehicles are being designed which are capable of high speed travel e.g. at speeds above 20 mph, on public roads. Accordingly the braking systems of such vehicles are required to comply with standards which apply to heavy road vehicles such as trucks. More particularly there is a requirement for the braking systems of such vehicles to have an ABS capability.
ABS becomes operative in response to an operator actuating a brake control, and upon the braking system sensing the slippage or impending slippage of at least one of the ground wheels relative to the ground. The ABS varies the braking force applied to at least one of the wheels between, in alternative periods, an applied state in which the braking force is applied, and a released state in which the braking force is released. The lengths of the applied state and released state periods and the magnitude of the braking force applied during the applied state periods, are determined by an algorithm which determines a notional deceleration of the vehicle and constantly monitors the speed of the at least one wheel to test the validity of the determined notional deceleration.
Thus for the ABS to operate optimally, it is essential that during the periods in which the braking force is released, the wheel accelerates due to its frictional contact with the ground, back towards a speed at which its angular velocity is close to the vehicle speed relative to the ground.
In the case of vehicles having heavy wheels, for example large tractor wheels with an outside diameter 2.0 meters or more and perhaps an axial length of 0.6 meter or more, such wheels will inherently have large inertia. In the case of vehicles such as dumpers, the rolling circumference of a wheel could be about 6 meters.
It has been found that during the periods in which the braking force is released, such wheels can fail to accelerate to anything like a speed at which their angular velocity is close to the vehicle speed due to their inertia. The problem is aggravated where there is a low coefficient of friction between the ground and the wheel, e.g. where the ground is icy, such that the wheels may slip relative to the ground during periods in which the braking force is released.
The effect on the operation of the ABS is that in monitoring the speed of a wheel during the periods in which the braking force is released, the ABS could make an incorrect assumption about the vehicle speed relative to the ground, based on its monitoring of the wheel speed to test the validity of the determined notional vehicle deceleration. More particularly, the ABS could perceive that the vehicle is slowing down at a rate faster than actually it is, and as braking progresses the error will increase. Thus, as indicated by the graph shown in FIG. 1, there is the possibility of the ABS being misled into determining that the vehicle speed has been arrested, when in fact the vehicle is skidding along the ground, e.g. a road surface, with one or more locked wheels.
In FIG. 1, the graph plots vehicle speed along the y-axis against time along the x-axis. The irregular curve indicated at 10 and the simplified line at 11, is a plot of the notional vehicle speed, or a reference, determined by the ABS, e.g. by sensing the speed of the two front wheels of the vehicle, and determining the average. This is a standard function of an ABS achieved in different ways.
The line 10 is indicative of a rate of deceleration of the vehicle the ABS perceives it is achieving, by applying the variable braking force according to an ABS algorithm. The curve indicated at 12 plots the actual vehicle speed relative to the ground as may be determined by an external sensing system. The shaded area of the graph between the two plots 11, 12 represents the error due to the inertia of the wheels, and the distance along the x-axis between where the two plots 11, 12 cross the x-axis indicates a period during which the wheel will be unwantedly and dangerously locked by the ABS.
Thus vehicles with high inertia wheels present a particular problem where it is a requirement to provide a braking system with ABS capability.